Jacketed,cooled crucible for crystallizing material



R. glbsssAu E R A ril 7, 1970' 'JAc'KETEn, 000mm CRUCIBLE FOR CRYSTALLIZING' MATERIAL Filed March 10, 19s? GEN.

INVENTOR RALPH G. DESSAUER ATTORNEY United States Patent 3,505,025 JACKETED, COOLED CRUCIBLE FOR CRYSTALLIZING MATERIAL Ralph G. Dessauer, Beacon, N.Y., assignor to International Business Machines Corporation, Armonk, N.Y.,

a corporation of New York Filed Mar. 10, 1967, Ser. No. 622,239 Int. Cl. B01j 17/18 US. Cl. 23-273 1 Claim ABSTRACT OF THE DISCLOSURE An apparatus in which a direct heating crucible is employed for the Czochralski method of growing single crystals from reactive materials such as silicon and germanium. The crucible takes the form of a shaped, single-turn high-frequency induction coil which is water cooled for thermal control.

FIELD OF THE INVENTION This invention relates to apparatus for processing fusible materials, and more particularly to a crystal growing apparatus for growing single crystals of materials, such as silicon, having relatively high melting points.

DESCRIPTION OF PRIOR ART One particular application of this invention is in the production of single crystals from materials such as silicon and germanium. Among the various methods of producing such single crystals of semiconductor materials, in one of the more successful methods, mono-crystalline semiconductor rods are produced from a melt of the material according to the Czochralski method. Such a method comprises melting the smiconductor material in a refractory crucible under a non-reactive atmosphere, dipping a singlecrystal seed in the melt surface, and slowly withdrawing the seed. As a result, the melt material solidifies on the seed in a crystal orientation determined by the seed. Regardless the process employed, it is necessary that the crystal compositions produced be, among other requirements, highly purified, i.e. contaminant-free, in order to obtain the desired characteristics of the semiconductor material employed, including any dopants therefor.

However, the foregoing described process is characterized by certain inherent disadvantages intrinsic in the apparatus presently employed. In this regard, it is known that semiconductor materials, and particularly silicon, is readily liable to contamination when in the molten condition, which is a hazard present in processes, such as that described, which utilize refractory crucibles, since the melt and the resultant crucible may be contaminated by impurities present in the refractory. For example, oxides of silica in such refractory crucibles are reduced by the molten material, particularly silicon, and transferred to the melt.

Efforts to eliminate such oxygen contamination, from refractory crucibles, have evolved in various diverse techniques of producing single-crystal semiconductor products. A typical result of such efforts is exemplified by the zonelevelling technique, which has somewhat reduced but not eliminated the dangers of contamination inherent in the continued use of refractory or quartz crucibles.

Further, efforts to reduce such contamination is reflected in US. Letters Patent No. 3,172,734 issued to R. W. Warren and directed to the use of water-cooled metal crucibles in the zone refining of semiconductor materials-According to the teachings of this patent, the

crucible, for the zone-refining method described, is made of a metal having a high electrical and thermal conductivity and constructed in the form of a hollow-walled ves- 3,505,025 Patented Apr. 7, 1970 sel having a fluid circulated through the hollow walls for cooling purposes. The metal crucible is then disposed for relative movement within an induction coil for appropriate melting of the semi-conductor charge.

SUMMARY OF THE INVENTION It has been discovered, in accordance with this invention, that the use of metal crucibles fabricated of high electrical and thermal conductivity metals which are, at least at low temperatures, inert to silicon, such as copper, silver, gold and the like, can be adapted for direct contact heating of melts of fusible materials from which, in one application, a single crystal of the material can be drawn. For such application the metal crucible is constructed as an integral part of a single-loop induction coil with the crucible maintained by suitable coolants, at a temperature which maintains the metal of the crucible substantially inert to the material being processed.

In general, the metal crucible may be fabricated or otherwise suitably shaped into the form of an open-top container having split or spaced side-walls which extend in an electrically discontinuous loop which thus defines with the container, a single-loop induction coil (i.e., a container/coil unit) when a high frequency current is impressed across the spaced, distal portions of the loop.

Cooling of the structure may be effected by suitable ap plication of a coolant about its external walls, as for example by conventional heat-exchange jacketing. In the preferred form, the provision for cooling is best obtained by fabricating the metal crucible of this invention, in the form of a double-wall structure with a suitable coolant, such as water, circulated through the hollow walls of the structure.

In operation, a charge of the desired material, e.g., a semiconductor material, such as silicon, is fed into the container of the container/ coil unit and a suitable high frequency current applied, as aforedescribed, of suflicient magnitude to melt the material in the melt-zone. A single crystal of the material, e.g. silicon, secured at the end of a draw-rod, is then placed in the melt. Normally, the drawrod is slowly rotated (ex. 10 rpm.) and withdrawn (ex. 0.5 to 6 inches/hour) in accordance with standard drawing techniques, from the melt to pull out a continuous single crystal of the material charged. Such a use of an inert metal crucible modified, in accordance with this invention, into an integral single-loop induction coil, eliminates the causes of contamination associated with the conventional use of refractory crucibles in crystal-pulling techniques.

Accordingly, it is an object of this invention to provide an improved apparatus for processing fusible materials.

Another object of this invention is to provide an improved crystal growing apparatus.

A further object of this invention is to provide an improved crystal growing apparatus in which the crystal is pulled from a melt of the same material.

The foregoing and other objects, features and advantages of this invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWING The figure is a diagrammatic drawing in perspective, and partly in section, of an improved crystal growing technique embodying the invention.

DESCRIPTION OF THE INVENTION Referring to the figure, a crystal growing apparatus is shown incorporating the invention in which the crucible 1 is fabricated of an electrically conductive metal, such as copper, as an integral part of a single-loop induction coil. In the particular embodiment shown, the crucible comprises a horizontally elongated, substantially open-top container of arcuate configuration. Such an arcuate configuration provides inclined portions of the bottom wall of the crucible 1 (which defines the charge receiving recess or cavity 2), which provides for cavity gravity feed of the charge 3 of the fusible material to be processed, which for purposes of illustration, is shown in the form of a rod of silicon. Optimally, if desired, provision for positive feed of the charge can be included in the form of a push-rod 4 connected to a conventional drive means, not shown, and which is cooperatively connected to the end of the charge, as by mere abutment, to feed the charge at the desired rate through the open-end 5 of crucible 1 to provide the working level of the melt 6 in the crucible cavity 2.

In accordance with the present invention, the crucible 1 forms an integral part of a high frequency inductive heating system for providing the necessary power input to form the melt 6. In the particular embodiment shown, the crucible 1 forms a single-loop induction coil by providing the crucible sidewalls 7 and 8, intermediate the ends of crucible 1, with respective continuations 9 and 10 with the ends of the continuations provided with terminal blocks 11 and 12 for connection to a high frequency current source at a frequency and power level suflicient to melt and maintain a pool of the material charged in the crucible. An R.F. generator 13 is shown for this purpose which, for a silicon charge, may, for example, be operated at a frequency of 450 kc. and a power level of 50 kw.

The continuation 10 forms an extension of an intermediate portion of the crucible sidewall 8 and is flanged or otherwise suitably fabricated into an external projection of the cmcible. In turn, the continuation 9 is also formed as a flanged extension of the crucible sidewall 7 which is looped over the crucible cavity 6 and into electrically insulating overlapping relationship with the continuation 10. The loop of the continuation 9, although shown as planar for purposes of close proximity to the melt to increase efliciency, may be, however, of any configuration provided it spans the open-top of crucible 1, and forms with it and continuation 10 a single-loop split coil across which a high frequency current may be impressed. In order to provide a positive electrical discontinuity between the overlapping portions of continuations 10 and 11, a layer 14 of electrically insulating material, such as Teflon may be inserted therebetween.

The crucible is constructed of metal which is inert to the material being processed, at least at low temperatures, and which, preferably, has high electrical and thermal conductivity with the latter characteristic of the metal facilitating the cooling of the crucible to a temperature which maintains the metal inert to the material and the melt at operating temperatures. Typical metals which may be employed for the crucible are copper, tin, silver and gold. However, it is to be understood that other metals may be used which have the indicated characteristics and which may be maintained inert to the processed material.

Temperature control of the crucible 1 can be readily obtained by flowing a coolant, such as water, about the external walls of the crucible by means of a heat-exchange jacketing and the like. In the particular form shown, the desired cooling is obtained by use of a double-Walled construction for crucible 1, which defines a fluid channel 22 between the inner wall 15 and outer wall 16 of the crucible, and between the interconnecting channel walls 17 and 18 in the sidewall continuations 9 and 10. A

suitable coolant, such as water, may then be circulated through the hollow walls of the crucible by means of an inlet tube 19 and outlet tubes 20 and 21 extending to the fluid channel 22 within the hollow wall construction of the crucible. The entrance and exits of the coolant are indicated by the arrows 26.

The portion of the upper sidewall continuation 9 spanhing across the crucible cavity, is provided with an access hole 23 to accommodate a draw-rod 24 which is provided with a chuck 25 to hold a seed crystal of the material to be drawn by vertical displacement of the draw-rod 24 as well as by rotation thereof, as indicated by the related arrows, 27 and 28, respectively, by conventional drive mechanism 29 well known and available in the art. In addition. the access 23 also provides interruption in the coil so as to weaken the RP. field and prevent excessive coupling of crystals being grown and chuck 25 and drawrod 24. The residual coupling with the drawn crystal functions as an after-heater to reduce dislocation densities.

Although not shown in the drawings, the crystal growing apparatus will normally be enclosed with a chamber provided with an atmosphere of a protective gas such as argon, helium, hydrogen, etc.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an apparatus for processing a fusible material, a crucible comprising:

a horizontally elongated open-top and hollow-wall metal container adapted for containing a melt of said material with a pair of sidewalls of said container having intermediate its ends a pair of continuations conductively connected to the sidewalls of the container and an RF. generator in closed circuit and defining with said container a single-loop induction coil wherein one of said continuations on said sidewalls externally projects from said container and the other of said continuations on a second of said sidewalls forms the loop spanning said container with the distal end-portion thereof disposed in overlapping and electrically insulating relationship with the first said continuation, and with said R.F. generator impressing a high frequency current across the overlapping continuations of said container of sufilcient magnitude to melt said material;

means for withdrawing a single crystal of said material from said melt; and

means for circulating a coolant through the hollow walls of said container.

References Cited UNITED STATES PATENTS 11/1960 Martin 23301 5/1963 Sterling 23273 FOREIGN PATENTS 11/1963 Germany.

US. Cl. X.R. 

